In this study, we report results on coating silicon solar cells with undoped and cobalt-doped zinc oxide (5%, 10%, 15%, and 20%) nanoparticles ranging in diameter from 46 nm to 87 nm, synthesized by a simple low-temperature precipitation method. The morphology and structure of nanoparticles have been characterized by transmission electron microscopy and X-ray diffraction spectroscopy. The average size of nanoparticles was found to increases with the increase in cobalt concentration. Doped and undoped zinc oxide thin films were deposited by a simple spin coating technique on silicon solar cells. Optical properties of the cobalt-doped zinc oxide layer have been investigated. We estimated the thin film stress by adding nanoparticles layers and found that the stress is lowest for the 139 nm cobalt-doped zinc oxide layer. Bandgap was found to increase with an increase in zinc oxide nanoparticle thin film thickness. Photoluminescence spectra show a blue shift in the near band edge emission peak with increase in thickness. Current-Voltage measurement confirms that there is an enhancement in power conversion efficiency as the thickness of zinc oxide nanoparticles varied from 58 nm to 139 nm. In general, the conversion efficiency shows an increasing trend up to 139 nm of thickness in silicon solar cells coated with doped and undoped zinc oxide nanoparticles. In addition, there is an enhancement of external quantum efficiency with an increase in thickness of zinc oxide layer. The optimal thickness of nanostructured zinc oxide film layer to enhance the efficiency of the silicon photovoltaic cell was experimentally determined to be 139 nm.